Lifting force regulated hydrofoil

ABSTRACT

The vessels weight is wholly or partially supported by one or more fully or partially submerged hydrofoil(s). The amount of lift provided by said hydrofoil(s) is regulated as a function of that lift itself. External variables such as trim angle and vessel velocity affect total lift, and this system compensates for those variables to provide a regulated lift without the need to directly sense any of the external variables. A typical control system comprises one or more sensory inputs, a method to translate the input into a control output, and an actuator that turns the outputs into actual movements. The only sensory input for this system is the same lifting force that is being regulated. The mechanical arrangement translates that lifting force into movements that produce negative feedback by varying the angle of attack of the hydrofoil, which in turn regulates the lift produced.

BACKGROUND OF THE INVENTION

Hydrofoil technology potentially offers improved efficiency for waterborne craft. Hydrofoils can be used to lift a vessel completely clear of the water surface so that the vessel is essentially flying, or they can be used to provide only a portion of the lift required for the vessel to float on the water surface.

The performance of the foil section used to construct a hydrofoil operates within a range of parameters that produce both lift and drag that vary in response to angle of attack. The performance of said hydrofoil is represented by the ratio of lift to drag with a higher ratio being desirable. For benefit to be derive from a hydrofoil, it must be operated in a manner that provides superior lift to drag ratio as compared to the unaided vessel by itself. A hydrofoil can achieve lift to drag ratios in the range of 0 to 150. A planing vessel typically might achieve a best-case lift to drag ration of around 6. Where the unaided planing vessel has an advantage is that its lift to drag ratio does not change significantly over a wide range of velocities. In comparison a hydrofoil has a narrower range of velocities where it can produce effective lift to drag ratios. This invention may be used to improve the portion of time, and the range of conditions, over which the hydrofoil can be considered a benefit.

Hydrofoil craft that completely rise above the surface present a lot of design challenges. The height must be enough to clear the wave action, but the height must be carefully regulated, and the pitch and roll of the vessel also must be controlled. This invention may be used to improve the control characteristics of vessels that fly above the surface by altering the range of performance of said hydrofoil(s) as compared with the raw unmodified hydrofoil performance. For example, if the total lifting force is capped at slightly higher than the weight of the vessel and occupant(s), then vertical acceleration of the vessel and occupant(s) in response to trim changes of said vessel can be better managed.

Some of these control problems cease to exist if the vessel remains on the water surface. But a new set of problems is then introduced. The lift must remain less than the total weight of the vessel or it will become airborne. The vessel being subject to wave action will change its trim angle. Hydrofoil lift provided may serve to destabilize the vessel. Again, this invention may be used to improve the control characteristics by altering the range of performance of said hydrofoils as compared with the raw unmodified hydrofoil performance.

The lift of a hydrofoil is proportional to its surface area, angle of attack, velocity squared and the foil section itself. It is thus difficult to design a hydrofoil that provides the correct amount of lift over a wide range of velocities and vessel trim angles, and it can be correspondingly difficult to control. The regulation of said lifting force in response to that force itself avoids needing to consider these independent variables.

At any given velocity the lift of a hydrofoil may be altered by changing its angle of attack, or using various possible lift modifying flaps or trim tabs that effectively alter the camber of the foil section. The system described by the claims varies the angle of attack to produce negative feedback that alters the performance of the hydrofoil compared with one unaided by such a system.

In attempting to regulate hydrofoil performance it is possible to operate the hydrofoil in a counterproductive manner. This system permits operating the hydrofoil in its optimal range irrespective of such variables as velocity and vessel trim angle.

An unaided planing vessel moving over a choppy surface loses performance (drag increases) as a result of pitching motions of the vessel. This situation is not improved with the use of a conventional fixed hydrofoil mounting, nor by using many of the existing hydrofoil control systems if the vessel remains in contact with the free water surface. In this case variations in vessel trim are coupled directly to the hydrofoil, and its response is to generate varying lift and drag in the similar fashion to the unaided vessel. Using this invention, changes of the hydrofoil lift and drag in response to trim angle changes can reduced, thereby improving overall efficiency in choppy conditions.

Unaided planing vessels experience a transitional hump when changing from displacement mode to planing mode. This hump requires a significant input of power. A hydrofoil can be used to reduce the magnitude of this hump and improve transitional performance. This invention automatically increases the hydrofoil angle of attack to its most optimal region (typically around five degrees) in order to be able to provide a best-case lift at the lowest resultant additional drag.

The elements of this system may be merged with the elements of additional control systems, such that the resultant behaviors may be combined. For example, a system that also regulates height of the vessel above the surface, or maybe adjusts the roll or trim angle of the vessel. Those other control systems are not within the scope of this invention, and do not impact the fundamentals of this invention. But this invention can be used to improve the net performance of the combined systems.

The performance of the control system may need to be adjusted to meet changing environmental conditions, for example sea state, or weight of the vessel and occupant(s).

BRIEF DESCRIPTION OF THE PRIOR ART

There are existing patents dealing with the control of hydrofoil lifting force. The claims described by these patents describe either how the lifting force is regulated, or the type of inputs that cause that regulation. A typical input that causes said regulation is vessel ride height above the free surface of the water. Another type of input used is vessel trim. These do not conflict with the claims of this invention where the input used is the lifting force itself. The method by which said lifting force is translated into a control movement using a mechanically resistive element and mechanical coupling also do not conflict with these existing claims.

A search of Hydrofoil+Control yields the following sample of ten patents, all of which can be dismissed easily as not conflicting with the claims.

U.S. Pat. No. 7,802,534: Self-propelled hydrofoil device with leverage-based control of drive foil. Manually coupled control actuated by the user.

U.S. Pat. No. 6,227,139: Control tab assisted lift reducing system for underwater hydrofoil surface. Control is provided by external force.

U.S. Pat. No. 5,054,410: Hydrofoil sailboat with control system. Roll is controlled by load in the shrouds of the sailboat.

U.S. Pat. No. 4,622,913: Hydrofoil flat control rod system. Control is provided by the force from an actuator and does not specify how that actuator force is derived.

U.S. Pat. No. 4,517,912: Hydrofoil control. Control force is a function of the depth of submergence of the control foil.

U.S. Pat. No. 4,228,750: Hydrofoil sailboat with control tiller. Control is achieved by a manually operated tiller.

U.S. Pat. No. 4,178,871: Automatic control system for hydrofoil craft. Control is derived from roll sensing signals.

U.S. Pat. No. 4,100,876: Hydrofoil fixed strut steering control. Steering control forces provided by a drive motor in response to steering input.

U.S. Pat. No. 3,994,253: Flap actuator control unit for a hydrofoil. Details the construction of a flap actuator and does not specify how control signals are derived.

U.S. Pat. No. 3,958,522: Automatic control system for hydrofoil craft. Control is derived from roll sensing signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 and FIG. 2 illustrate how a mechanical mounting system may be built that translates changes in hydrofoil lifting force into changes of hydrofoil angle of attack that produces some quantity of negative feedback on said lifting force.

DESCRIPTION OF THE INVENTION

The beam shown in the drawing was implemented as a hinged plate that also incorporated the mechanical connection to the hydrofoil strut. As the lift produced by the hydrofoil increases the resistive element compresses, this causes the plate to rotate clockwise about its pivot point and that causes the angle of attack to decrease, which in turn decreases the lifting force. The plate will rotate about its pivot until the hydrofoil lifting forces equal the resistance force from the resistive element measured coaxially and perpendicularly through the center of lift of the hydrofoil.

Variation of the characteristics of the resistive element can be used to change the performance of the hydrofoil. A simple gas spring was observed to produce ideal results for the sailboard application but may not necessarily produce the best results for other types of vessel.

As the drawings show, most of the lifting forces are transferred directly to the top mounting point of the resistive element. This can aid the overall stability of the system and provide a more direct coupling to the occupant standing on the top surface of the vessel. As applied to a different kind of vessel such as a powerboat or ship, this effect could be significant.

The claims were tested as applied to a sailboard. The same design can be applied to other types of vessel but may need some adjustment to perform optimally in those cases.

The implemented design is shown by these two drawings. The first implementation also featured a secondary stabilizer hydrofoil added to reduce pitching movements of the vessel in choppy water. Subsequently a second implementation was made that eliminated that secondary stabilizer hydrofoil.

Various types of resistive element were considered in the design phase that included the following.

-   -   1. Conventional compression springs (concentrically mounted over         a strut).     -   2. Compression gas spring.     -   3. Compression gas strut with a concentrically mounted         compression spring.     -   4. Tension spring (using a different mechanical arrangement).     -   5. Torsion spring (using a different mechanical arrangement).     -   6. Tension gas spring (using a different mechanical         arrangement).

Each of these required a different mechanical arrangement to facilitate the correct negative feedback as well as other mechanical constraints of simply mounting the hydrofoil to the vessel. Any of these would have worked, but in consideration of the desired behavior of the sailboard application the compression gas spring was selected. This choice satisfied some other basic constraints to built something that was not only strong enough to withstand all the forces, but also light, and which could be contained within the available space.

As built the implementation used two identical gas springs with the center between the gas springs being coaxially located with the center of lift for the hydrofoil. This was done for several reasons, but these two springs could be considered effectively as a single spring located at the geometric center between the springs. The designer has the option to vary this location and the pressure within the gas springs.

Another advantage of the hinged plate is that lateral forces from the hydrofoil strut are transmitted to the vessel without impeding the vertical motion of the plate and the attached hydrofoil.

When considering the characteristics of the gas spring used, its compressive force is not exactly constant, but increases by about thirty percent at full compression. The mechanical arrangement of the drawings causes the hydrofoil itself to move away from the pivot point by about thirty percent also at full compression, which increases the lever arm of the hydrofoil lifting forces. The net result is the lifting force is regulated to being approximately constant.

The observed behavior was that the hydrofoil produced lift that was a significant portion of the total weight of the vessel and occupant, but not enough to raise the vessel above the water surface. The gas springs were observed to compress and expand in response to vessel trim angle and velocity, thereby changing the angle of attack of the hydrofoil in order to maintain almost constant lifting force. At low velocities high angle of attack (approximately five degrees) gave maximal lift performance for the NACA 63A311 foil section with a corresponding lift coefficient of around 0.7. At higher velocities the angle of attack reduced to yield smaller lift coefficients that produced approximately the same total lift. Similarly, as the trim angle of the vessel changed, the angle of attack of the hydrofoil was maintained by compression and expansion of the gas springs to produce the same lift.

The gas spring pressure was set to provide approximately 80% of the weight of the vessel and occupant as direct lift as measured at the hydrofoil itself. This figure represents a tradeoff between stability and controllability of the vessel and performance. This can easily be adjusted by changing the internal pressure of the gas spring. The hydrofoil typically operates in a more efficient manner than the planing surface or displacement mode of the vessel. As measured by the lift to drag ratio. As the angle of attack of the hydrofoil decreases the lift to drag ratio may fall from the optimal lift/drag range and eventually the advantage given by the hydrofoil in comparison to the planing surface will disappear. Typically, the range of velocities covered starting from the point full lift is developed and extending to the point of diminishing returns is a factor of between three and four times but depends upon the foil section selected.

The designer has the option to adjust the surface area of the hydrofoil and its foil section to control the velocities at which these end points occur. The designer needs to pay attention to the placement of the resultant lifting force in relationship to the center of mass of the vessel. Different locations will yield a different overall behavior. Additionally, the lateral forces associated with the supporting strut or mast may need to be considered. The center of lift of the main hydrofoil may need to be in a different location from the center of lateral resistance of the vessel. Techniques exist for this to happen that are beyond the scope of this patent, but the second sailboard implementation built displaced the hydrofoil forwards from the vertical strut (fin) using a cantilevered beam (fuselage). 

What is claimed is:
 1. These claims describe a hydrofoil control system comprising elements of input sensing, functional mapping between input and output, and output actuation that control the lifting performance of one or more hydrofoils.
 2. These claims are applied to any waterborne vessel with or without occupant(s) and powered by any means. These claims may be applied equally well to an unmanned drone vessel.
 3. Some or all the weight of said vessel and said possible occupant(s) being supported by one or more fully or partially submerged foils. In the case where there is acceleration in any direction, the force generated may exceed the weight of said vessel.
 4. Said vessel may remain predominantly in contact with the water surface or may fly above the water surface.
 5. A method by which the lifting force produced by the hydrofoil(s) is coupled to a mechanically resistive element that both opposes said force applied to it and produces a precise movement in response to that force.
 6. A mechanical coupling that transforms said movement within the mechanically resistive element into a control output that varies the angle of attack of said hydrofoil.
 7. Said change in angle of attack of said hydrofoil regulates the lift produced by said hydrofoil as stable negative feedback.
 8. The precise magnitude of said negative feedback being defined both by the specification and characteristics of said resistive element and by the design of said mechanical coupling, and which together form said functional mapping between input sensing and output actuation.
 9. The properties of said mechanical resistive element may be adjusted to alter the overall performance of said control system. 